Objectives: a) We plan to obtain the highly active chicken cell R1 antigen isolated by us in physicochemically homogeneous form and to determine its overall chemical composition. We want to define its immunodominant groups more precisely and will establish not only their relation to the human MN antigens and the N precursor antigen in mouse TA3 mammary adenocarcinoma, but we will also assess the relation of the R1 antigen to the cell membrane component(s) isolated from fibroblasts of chicks, homozygous for R1, to which an oncogenic RSV-B virus apparently attaches as first step in its infectious cycle. b) We plan to solubilize and then isolate and characterize physically, chemically and biologically the active substance(s) in the fibroblast membrane preparations of R1R1 chicks which we have recently obtained. Appropriate control chick fibroblasts will be run in parallel. c) We will determine if the substances mentioned under a) and b) are able to inhibit the infection of susceptible chicken fibroblasts by Rous sarcoma viruses. If the results are positive, we will test these substances for their protective effect of chick embryos. Finally, we intend to stimulate cytotoxins against TA3 and possibly against human breast cancer cells by these chicken cell membrane antigens.